The present invention is related to the field of cutting plastic tubes which are to be butt welded together. Such a field includes the attempt to provide a sterile connection between two tubes for medical or scientific purposes. For example, the invention would relate to the sterile docking in continuous ambulatory peritoneal dialysis. U.S. Pat. Nos. 4,369,779 and 4,610,670 are generally directed to such types of systems. A commercial practice of those patents is in a device marketed under the name SCD.
Thermoplastic tubing can be butt welded together by any number of means. In one such method the tubes are positioned side-by-side and severed by a hot knife (welding wafer). The severed tubes are then re-positioned while still in contact with the wafer, the wafer is then removed, and the tube ends are pressed together to make the weld. Since the wafer must reach temperatures well above the melting point of the tube material, the melting tube typically leaves a residue on the side of the wafer. This residue must either be removed or a new wafer used for the next weld. This residue problem results in the need for a number of costly parts in the welding device:
cleaning tools & solvents or new wafers; PA1 wafer feed mechanism; PA1 wafer alignment system; PA1 thermocouple temperature control; and PA1 fume control system.
As taught by U.S. Pat. No. 4,610,670, thermoplastic tubes containing fluid can be welded by flattening the tubes. Although the same amount of pvc is cut when welding either round or flattened tubing, the wafer takes less time to cut flattened tubing so the rate of heat loss from the wafer is higher. This effect results in a very significant temperature drop as the tubes are welded.
If sterility is to be assured in the resulting weld this temperature drop must be minimized. The makeup energy required must come from one or both of two sources:
1. the transfer of energy stored in the wafer, wafer holder, etc. or
2. electrical energy from the built-in resistance heater in the wafer.
Experiments have shown that the wafer gets most of the make-up energy during the weld from heat stored in the wafer and wafer holder and not from the conversion of electrical energy to heat via the resistance heater.
The only existing welding wafer is the one used on the DuPont "SCD". It consists of an etched type 302 stainless steel serpentine resistance element folded into a copper sheath. The circuit is insulated for the copper and the unit is held together by a high temperature adhesive (DuPont Pyrolux).
This existing technology is inadequate in a number of areas:
1. Wafers must be replaced prior to making each weld.
2. Wafer construction techniques and material limit welding temperatures to approximately 575 degrees F., making it impossible to weld polyurethane tubing for example.
3. The wafer has very little thermal mass so that welding tubes or even breathing on the wafer during a weld could cause a severe temperature drop, possibly resulting in a non-sterile weld.
4. The wafer does not address the special problems associated with welding flattened tubing.
5. The wafer is mechanically weak and therefore is very difficult to clean and re-use.